Saturday, May 16, 2015
Well, I thought it was Isoperla nr. holochlora -- but the thinking has changed
(Caution: scholar at work.)
I was in touch with Steven Beaty yesterday, exchanging e-mails on a number of issues. In passing, I sent him this photo of one of the nymphs that I collected on Thursday, asking for confirmation that this was, indeed, Isoperla nr. holochlora. I was concerned because of the size. In his descriptions of Isoperla Perlodids ("The Plecoptera of North Carolina," p. 23), he gives the size range of I. holochlora as 11-44 mm; the range for I. nr. holochlora, on the other hand, is 9-10 mm. This nymph measured 13. His response was that rearing has shown this nymph (the one in the picture at the top of the page) to be Isoperla holochlora, but, at the moment, his group labels these nymphs "Isoperla holochlora, A." They are still keeping tabs on this type of nymph because of the dark abdomen, on which "stripes" are difficult to detect. So, at the moment, we seem to have two types of Isoperla holochloras. The common one that we see,
and type "A."
He also noted, as I have seen, that both types can co-occur (i.e. show up in the same stream). But, type "A" is the first to appear and the first to mature. (Actually, the only place that I've seen both types of nymphs is the Rapidan River [maybe Entry Run in Greene County??]; in most streams that I visit, I only see the "common" nymph for this species.)
I decided I had best take a very close look at his descriptions for I. holochlora and I. nr. holochlora (pp. 23-24 in his key). There we find this.
I. holochlora -- nymphs 11-14 mm; apex of lacinia narrower at base, with row of setae below subapical tooth; pale area anterior to median ocellus open to labrum, an oval spot lateral to each side of the ocellar triangle; ocellar spot narrowly, often barely, open behind; dark, abdomen with longitudinal stripes, median stripe narrow and interrupted, often obscured. Nymphs are common and abundant in the Mountains and Piedmont from March through August. (I've highlighted the key features in Bold.)
Let's look at both of our nymphs to see how well they match this description.
1) "pale area anterior to median ocellus open to labrum." Yes. In fact there's a wide area of that spot that touches the labrum.
2)"an oval spot lateral to each side of the ocellar triangle." Check. (see the green arrows)
3) "ocellar spot narrowly, often barely, open behind." Actually, on this nymph, a good portion of that spot is open behind.
4) "dark abdomen with longitudinal stripes, median stripe narrow and interrupted, often obscured."
Yes. And the lateral stripes are "wide" and bordered by thin, pale stripes.
5) Size? I'm not sure about this. I've looked at the specimens of this type of nymph in my reference collection. Those that are fairly mature are 8-9 mm. I'll check this carefully in a month or so when I start seeing the mature nymphs for this year.
1) "pale area anterior to median ocellus open to labrum." Not sure that I can agree. There is a thin point that points in that direction, but it sure doesn't touch.
2) "an oval spot lateral to each side of the ocellar triangle." Yes. Those we can see.
3) "ocellar spot narrowly , often barely, open behind." Can't agree. The spot on this nymph is surrounded by a dark border -- i.e. it's "closed."
4) "abdomen with longitudinal stripes, median stripe narrow and interrupted, often obscured." Actually, all three stripes are fairly "obscured," but you can see them if you look at the top abdominal segments.
5) Size? Good match. The nymph I found yesterday was 13 mm, and it was fairly mature.
The scholar in me isn't real pleased with the way in which either nymph fits the description. But to be fair, this document is currently being revised.
I. nr. holochlora -- nymphs 9-10 mm; apex of lacinia narrower than base, with row of 5-6 robust setae below subapical tooth and finer setae approaching base; head with pale M-pattern and with an oval spot lateral to each side of the ocellar triangle; ocellar pale spot absent; lateral abdominal stripes wide and with pale borders, median stripe narrow, sometimes obscured. Relatively common and often abundant primarily in the Mountains. Nymphs are collected from winter into late spring. (Again, the Bold highlighting is added by me.)
I don't think I've ever seen a nymph that fits this description. "head with pale M-pattern"? No. "ocellar pale spot absent"? No. That spot is clearly present in all of the photos posted above. Now, we do see abdominal stripes of the type that are noted (lateral stripes wide with pale borders) in the common type of I. holochlora, and the size is right for that type of nymph, but it sure isn't a match. Let me add that Beaty now feels that I. nr. holochlora as described in his key is really a new species, not a holochlora at all.
So there you have it. We're up to speed on the issue. Beaty has looked at my photos and confirms that both of these nymphs are I. holochlora. So, despite my reservations, that's what I'll call them, unless I hear something else.